In the manufacture of pressure-sensitive recording papers, better known as carbonless copy papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor is normally coated on the back side of the front sheet of paper of a carbonless copy paper set. This coated back side is known as the CB coating. In order to develop an image or copy, the CB coating must be mated with a paper containing a coating of suitable color developer, also known as dyestuff acceptor, on its front. This coated front color developer coating is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor. Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dyestuff precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff, or dyestuff precursor, then reacts with the color developer in the areas at which pressure was applied, thereby effecting the colored marking. Such mechanism or the producing technique of pressure sensitive recording papers is well known.
Heretofore, most CF coatings were applied as an aqueous system. The color developer may typically be dispersions of clay, phenolic resin, or zinc salts of an organic acid in water with a small amount of binder. They are typically coated by any one of a number of well known aqueous coating methods onto the paper substrate. Moisture is removed by passing the substrate through ovens. The finished coating is rough and usually requires calendering. There are numerous disadvantages to this procedure.
1. The aqueous coating equipment required is very expensive. PA1 2. Large amounts of energy are expended to dry the coating. PA1 3. This method is an off-line (full coat) operation with regard to the manufacture of multicopy form sets, and is more expensive than on-line (partial coating) printing operations. PA1 1. The ink has a distinct and strong odor characteristic of phenolic resins and aliphatic solvents. PA1 2. The nature of concentrated phenolic resin solutions makes these inks highly tacky. PA1 3. The inks possess poor image stability. PA1 4. The resulting ink film is fugitive; it migrates through the paper substrate producing poor aging characteristics.
The limitations and costs inherent in aqueous CF coatings led to the development of CF inks incorporating phenolic resins. These inks are essentially high-boiling-point aliphatic solvents containing a large amount of a phenolic resin dissolved therein. Although they are appliable by means of printing press and require none of the expensive equipment associated with aqueous coatings, they suffer from the following inferior physical properties.
The latter point represents the most serious drawback of these inks using organic solvent. Ideally one would prefer that the solvent drain off leaving the phenolic resin as an immobile film. However, when phenolic resins in compatible solvents are used, there is no physical reason for such a separation of resin and solvent. In practice, the migrating solvent carries phenolic color developer into the sheet and offsets onto adjacent sheets. This movement causes excessive discoloration of CF-CB sets and limits their shelf life. There is no true conversion from a mobile ink to a solid immobile film.
Coatings dried by evaporation, e.g., those applied by the flexographic or gravure processes, or by ultraviolet cure have been formulated to immobilize the CF film. The solvent coatings, however, require expensive solvent recovery equipment or consume large quantities of expensive solvents with related solvent vapor emission problems. The expense of such an operation is prohibitive. The formulation of the UV curable inks requires extensive press modification, costly ingredients, and a high percentage of non-active materials which reduces the resulting films' usefulness. These two types of ink do not represent economically viable solutions.
No means has previously been available for the on-line printing operation of a CF ink to obtain an immobile solid film without sacrifice of imaging potential.